The present invention relates to an exposure apparatus and a method for manufacturing the same, which has an excimer laser light source, a higher harmonics laser light source, a mercury lamp light source or the like, each radiating light having an ultraviolet range of a wavelength of, for example, 300 nm or shorter. The present invention also relates to an optical device for a projection optical system or an illumination optical system for use with such an exposure apparatus, and to a method for cleaning such an optical device.
An exposure apparatus for exposing an image of a pattern of a reticle (a photomask or the like) onto a photosensitive substrate through a projection optical system has been employed in a lithography process for manufacturing semiconductor elements, liquid crystal substrates, and so on. Recent years, developments have been performed to make semiconductor integrated circuits finer and finer, and in order to comply with such finer integration of semiconductor circuits, there has been attempted to make shorter an exposing wavelength of a light source for use in a lithography process.
At a current time, an exposure apparatus has already been developed which uses a KrF excimer laser having a wavelength of 248 nm as a light source for a stepper. For example, a higher harmonic wave of a wavelength variable laser such as Ti-sapphire laser, etc., a quadruple harmonic wave of a YAG laser having a wavelength of 266 nm, a fivefold harmonic wave of a YAG laser having a wavelength of 213 nm, a mercury lamp having a wavelength close to 220 nm or a wavelength of 184 nm, and an ArF excimer laser having a wavelength of 193 nm draw attention as a candidate for a light source having a shorter wavelength.
For conventional exposure apparatuses which use g-rays, i-rays, a KrF excimer laser, or a mercury lamp emitting light rays having a wavelength close to 250 nm as a light source, emission spectral rays of such a light source do not overlap with an absorption spectral region of oxygen, so that they do not cause any decrease in efficiency of light utilization due to absorption of oxygen and do not suffer from any disadvantage resulting from the generation of ozone due to the absorption of oxygen. Therefore, those exposure apparatuses can basically be used for exposure in ambient atmosphere.
On the other hand, however, for a light source such as an ArF excimer laser, emission spectral rays overlap with an absorption spectral region of oxygen, so that a decrease in efficiency of light utilization may be caused by the adsorption of oxygen, and the disadvantage may also result from the generation of ozone due to the absorption of oxygen. For instance, if it is supposed that a transmittance of an ArF excimer laser light in vacuum or through an inert gas such as nitrogen or helium is 100%/m, the transmittance is decreased to approximately 90%/m, on the one hand, when the light is in a free-run state, i.e., in a natural emission state, that is, the light source is an ArF broad-banded laser, and it is decreased to approximately 98%/m, on the other hand, even when there is used an ArF laser with the spectral width narrowed and so narrow-banded as to avoid rays of absorption of oxygen.
It is considered that the decrease in transmittance is caused due to the absorption of light by oxygen and an influence of ozone generated. The generation of ozone is considered to exert an adverse influence upon the transmittance of light (i.e., efficiency of light utilization) as well as to cause a deterioration in performance of devices due to a reaction with a surface of an optical material or other parts and to cause a pollution of environment.
For the above-mentioned conventional exposure apparatuses having a light source such as an ArF excimer laser in the configuration as described above, it is well known that the entire area of a light passage is required to be filled with an inert gas such as nitrogen or the like, in order to avoid a decrease in transmittance of light and a generation of ozone.
As a result of various exposure experiments using a projection exposure apparatus with an excimer laser light source installed therein and having a relatively large field size, a new phenomenon has now been discovered in that the irradiation of an illuminating light in an ultraviolet region having a wavelength range of, for example, 350 nm or less (e.g., KrF excimer laser having a wavelength of 248 nm or ArF excimer laser having a wavelength of 193 nm, etc.) dynamically fluctuates transmittance or reflectance of an optical element in a projection optical system or a coating material (e.g., a thin film for a reflection preventive film, or the like) for the optical element in the projection optical system. It is further found that this new phenomenon fluctuating the transmittance of light dynamically can be caused to occur in substantially the same way as not only in the case of an optical element disposed in the projection optical system but also in the case of an optical element disposed in an illumination optical system for illuminating a reticle. Moreover, likewise, it is also found that the such phenomenon is caused to occur in the case of an optical element in a light sending system leading the illuminating light leaving from a light source disposed under a floor of a clean room to an illumination optical system installed in the main body of the exposure apparatus, and in the case of the reticle (a quartz plate) itself.
In addition, the such phenomenon is considered to occur, for instance, due to the attachment of impurities contained in a gas (e.g., air, nitrogen gas, etc.) present in a space of a projection light passage or an illumination light passage, molecules of organic substances departing from adhesive or a filling material, etc., for use in fixing optical elements to a barrel, impurities (e.g., water molecules, hydrocarbon molecules, or other substances diffusing the illuminating light) derived from the inner wall) derived from an inner wall of the barrel (e.g., a coated surface for reflective prevention, etc.), or otherwise, or due to the entry (floating) of such impurities or otherwise into the illumination light passage. As a consequence, a serious problem is considered to be caused such that the transmittance or reflectance of the projection optical system, the illumination optical system, and the light sending system fluctuates to a great extent for a relatively short period of time.